The cellular slime mold, Dictyostelium discoideum (NC4), grows as a single-celled amoeba. After food is exhausted, the cells aggregate forming a multicellular organism in which two predominant cell types, prespore and prestalk cells, differentiate. Utilizing the appearance of specific antigens and enzyme activities, it was previously shown that preaggregating cells cannot form clumps and cannot differentiate into prespore cells under liquid shaking culture conditions in media containing glucose-albumin-EDTA and cyclic AMP (GAC). Post-aggregating cells, however, can form clumps and differentiate into prespore and prestalk cells in GAC. Differentiation is dependent on cAMP and cell contact. We have investigated expression of cell type-specific mRNAs under these conditions. Preaggregating cells transferred into GAC did not express prespore or prestalk mRNAs. By contrast, aggregating cells transferred to GAC expressed large amounts of both prespore and prestalk mRNAs. When clump formation was inhibited by rapid shaking, aggregating cells in GAC expressed only very low levels of prespore and prestalk mRNAs. Addition of ammonium sulfate to GAC could partially restore prespore mRNA expression and completely restore prestalk type 1 mRNA expression without clump formation. These results show that expression of both prestalk and prespore mRNAs require a specific differentiation step which does not proceed in GAC media. Additionally, at least part of the effects of clump formation (cell contact) on prespore and all of the effects on prestalk cell differentiation can be replaced by addition of (NH4)2SO4.